PURPOSE:To set quickly a signal at a fixed level and to reduce the transmission time without having a long idle time by using a thyristor starting circuit to a phase control circuit of the thyristor and an idle signal generating circuit. CONSTITUTION:A direct current 1 is rectified by a thyristor 2, etc., and a power transistor 5 is switched to inject signals to an injecting circuit 6. With input of a transmission start signal (a), a phase control circuit 8 starts triggering to gates G1 and G2 of the thyristor 2. At this time point an FS signal generating circuit 10 delivers a signal (ic) corresponding to an idle signal (ib). Therefore the DC power supply voltage of a signal injecting power circuit PC has a rise at a charging time point t3. This period t3 is extremely short compared with a discharge characteristic period, and therefore the time for the trigger signal of one time suffices for the idle signal generating time t2. In this case, the injection signal e1 to the power line is set almost instantaneously at a fixed level.